The present invention relates to a niobium oxide sintered compact, a sputtering target formed from the sintered compact, and a method of producing the niobium oxide sintered compact.
In recent years, ReRAM which utilizes the significant change of electrical resistance caused by voltage application draws attention as an alternative to a flash memory, and known is the use of oxides of transition metals such as nickel, titanium, tantalum and niobium, as the resistance change layer of the ReRAM; particularly oxides (suboxides) having a composition which deviates from stoichiometry (refer to Patent Documents 1 to 3). For example, Patent Document 1 discloses a niobium pentoxide (Nb2O5) as the resistance change layer provided in the ReRAM.
A thin film formed from a niobium oxide is normally formed via the sputtering method. For example, Patent Document 4 relates to an example of a niobium pentoxide and discloses a niobium oxide sputtering target which is produced by subjecting a niobium pentoxide powder to pressure sintering by way of hot press or other means.
Meanwhile, when producing a sintered compact for use as a target, the simplest production method is to match the composition of the intended sintered compact and the composition of the raw material. Nevertheless, niobium oxides that are generally available as a single phase are the three types of NbO, NbO2, and Nb2O5. For example, a suboxide such as NbO2.2 is not available as a raw material, and therefore it was not previously possible to produce a sintered compact having a composition of NbOx (2<x<2.5). Note that, among NbOx (2<x<2.5), Nb12O29 (≈NbO2.417) is available, but it is difficult to acquire because it is not distributed on the market.
Thus, considered may be a method of producing a sintered compact having a composition that differs from the composition of the raw material based on reaction sintering (method of simultaneously performing synthesis and sintering). However, when a metal (Nb) and its oxide are mixed and sintered to obtain the intended composition (valence of oxygen) and prepare a sintered compact, there are problems in that an unreacted substance may remain or numerous small holes are formed in the sintered compact. In addition, when producing a large sintered compact, there is a problem in that the in-plane density of the sintered compact is not uniform.